Conventional specular techniques for inspecting printed wiring boards often provide a large number of false indications. In such techniques light is reflected from small regions of the board and the intensity of the reflected light is detected to indicate the presence of copper circuit material or substrate. However, if the flatness of the reflecting surface varies even slightly from what is expected, the light rays may be redirected so that no reflected light reaches the detector. In such cases the detector fails to detect the presence of the reflecting material.
Alternative systems have illuminated the substrate either from below or at a region away from the region to be inspected so that the printed foil creates shadows which indicate its presence. Such systems are typically not successful when used with boards employing large areas of copper which tend to block the light. Moreover, foil provided on the bottom of the board is likely to interfere with lighting from below the board and provide false indications when the top of the board is being viewed.
In one approach a laser has been used to illuminate the substrate at a first wavelength and cause certain substrate materials to fluoresce at a second wavelength. The foil appears as a shadow against the fluorescent substrate. This technique is relatively complicated and requires that fluorescent materials be employed. Such materials affect the chemical composition of the board.
In another relatively complex system the board is placed in a vacuum chamber and an electric field is applied between the circuit material and a transparent electrode spaced above the board. The air between the foil and electrode is ionized to provide a glow indicating the presence of foil.